Composite boron filaments with matrix overcoat

ABSTRACT

A BORON FILAMENT HAVING A THIN, PROTECTIVE LAYER OF SILICON CARBIDE IS FURTHER PROVIDED WITH AN OVERCOAT OF A MATRIX MATERIAL INCLUDING PARTICULARLY THE LIGHT METALS SUCH AS ALUMINUM, MAGNESIUM, TITANIUM AND THE ALLOYS THEREOF, TO THEREBY PROVIDE A FILAMENT WHICH IS NOT ONLY CHARACTERIZED BY HIGH STRENGTH BUT ALSO BY IMPROVED UTILITY IN THE PRODUCTION OF FIBER-REINFORCED ARTICLES.

Jan, 19, 1971 M. BAscHE ErAL i COMPOSITE BORON FILAMENTS WITH MATRIX OVERCOAT Filed Feb. 24. 19e? United States Patent 3,556,836 COMPOSITE BORON FILAMENTS WITH MATRIX OVERCOAT Malcolm Basche, West Hartford, Conn., Roy Fanti,

Springfield, Mass., and Salvatore F. Galasso, Manchester, Conn., assignors to United Aircraft Corporation,

East Hartford, Conn.

Filed Feb. 24, 1967, Ser. No. 618,514 Int. Cl. B32b 15/04; B44d 1/42 U.S. Cl. 117-71 4 Claims ABSTRACT OF THE DISCLOSURE A boron filament having a thin, protective layer of silicon carbide is further provided with an overcoat of a matrix material including particularly the light metalsl such as aluminum, magnesium, titanium and the alloys thereof, to thereby provide a filament which is not only characterized by high strength but also by improved utility in the production of fiber-reinforced articles.

BACKGROUND OF THE INVENTION It is known that filamentary amorphous boron may be produced by pyrolytic techniques wherein the boron is deposited on a resistively heated wire which is drawn through a gaseous reactant stream consisting of boron trichloride admixed with hydrogen.

Early investigations quickly revealed the potential utility of these fibers in the production of fiber-reinforced articles having improved physical properties. In order to exploit the high strength properties of the filamentary materials, however, it is necessary to gather the fibers together in such a way that the load imposed thereon is I. distributed over the entire fiber bundle. One method of effecting this result is to encase the fibers in a matrix material which will deform plastically.

The reactivity of boron has heretofore not only limited the choice of matrix materials with which this fiber is usable, but also has additionally limited the temperatures at which the articles wherein it is used are fabricated and operated. In a copending application entitled Composite Boron Filaments by Malcolm Basche, Roy Fanti and Salvatore F. Galasso, Ser. No. 618,513, filed Feb. 24, 1967, and now abandoned, there has been disclosed a composite filament comprising boron coated with a thin protective layer of silicon carbide. The composite filaments have been found to be compatible with a variety of preferred matrix materials including aluminum, magnesium, titanium and alloys thereof.

One of the paramount problems in obtaining high strength, high modulus fiber-reinforced articles involved the actual incorporation of the fibers into the matrix material to provide the desired end product. In the most common filament sizes particularly, breakage of the filaments in operations such as winding is a relatively common occurrence, particularly wherein short radius bends are effected as one fine filament is overlaid over another in a different winding direction. And even in operations where the composite filaments do not actually break, the short radius bend may rupture or stress the silicon carbide coating and, by destroying its integrity, render the boron substrate prone to degradation through a substratematrix interaction.

SUMMARY OF THE INVENTION The present invention relates to amorphous boron filaments having a thin, protective layer of silicon carbide and being further provided with an overcoat of a ductile matrix material. It contemplates silicon carbide-coated boron fibers provided with an overcoat comprising the ICC lightweight materials including aluminum, magnesium, titanium and alloys thereof. Not only are these fibers less prone to damage and breakage than the basic boronsilicon carbide fibers, but, in some instances, they have exhibited a totally unexpected strength increase which is not predicted from the fundamental characteristics of the individual material components. The synergistic effect is dramatically shown by an ultimate tensile strength increase from about 460,000 p.s.i. for the basic composite to an average strength of about 512,000 p.s.i. for the filament with an aluminum overcoat. Theoretical considerations would, of course, dictate a decrease in the ultimate tensile strength through the addition of a material such as aluminum. Furthermore, with the ductile, conductive overcoat localized mechanical stresses and thermal effects are distributed over substantial fiber areas. Still further, in the formation of fiber-reinforced articles the matrix overcoat provides not only good bond strength between the fiber and the matrix overcoat but appears also to provide better bonding with any additionally applied matrix material, including the resins. Since the overall strength of the fiber-reinforced article is dependent upon the filament-matrix bond strength, the improvement in the physical properties of the end product is evident.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simple sketch, taken in elevation, of apparatus used in the production of the silicon carbide coating on the filaments of the present invention.

FIG. 2 is a sketch, taken in cross-section, of the melt apparatus utilized in the formation of an aluminum overcoat on the composite boron filament.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As seen in FIG. l, the silicon carbide coating is produced on a resistively heated boron filament 2 which is drawn downward through a reactor 4 comprising a tubular containment vessel 6, having dual gas inlets 8 and 10 at the upper end of the reactor, and a single exhaust port 12 at the lower end thereof. Cooling hydrogen is fed to the reactor through inlet 8, and inlet 10 is used for the introduction of the reactant gas mixture including methyldichlorosilane (CHaSiHClZ), hydrogen and methane. The containment vessel may be formed of Pyrex, although a number of other materials including Vycor and quartz will be found satisfactory. The gas inlets 8 and 10 and the exhaust 12 penetrate and are electrically connected to the metallic end plugs 14 and 16 which provide the end closures for the containment vessel and, also, provide convenient means by which power may be supplied to the wire for resistance heating purposes.

Although the end plugs may readily be seen to differ in overall configuration, they both incorporate a number of common features. They are each formed to provide a Well 20 and 22, respectively, for containing a suitable conductive sealant 24, Such as mercury, which serves the dual purpose of providing a gas seal around the wire where it penetrates the end plugs, and further providing electrical contact between the moving wire and the respective end plugs which are in turn electrically connected through the tubes 10 and 12 and the leads 26 and 28 to a suitable DC power source 30. A variable resistance 32 is provided in the external circuit to permit regulation of the power supplied to the wire and, hence, temperature control thereof. The upper plug 14 is provided with a peripheral groove 34, which communicates with the mercury well 20 through the passageway 36, to provide peripheral sealing around the plug. Sealing between the end plug 16 and the lower end of the containment vessel 6 is provided by mercury contained in an annular well 38.

The respective plugs are further each formed with a 3 centrally-oriented orifice 40 and 42 which is large enough to accommodate the free passage of the Wire therethrough but which, in combination with the wire, is small enough to retain the mercury, through surface tension forces, in the respective Wells.

The hydrogen admitted through the inlet 8 enters the reactant chamber immediately adjacent the wire inlet and is used primarily for cooling purposes at the end plug 14. The reactant gases enter the reactant chamber in an enlarged chimney portion 50, reverse flow therein, and enter the tubular member 6 at opening 52.

Subsequent to the formation of the silicon carbide layer, the laments are provided with a matrix overcoat of the desired composition. In the case of aluminum, the overcoat has been provided by processing the fiber in apparatus such as that disclosed in FIG. 2. A Crucible 60 formed of alumina is'suitably positioned within a furnace including brickwork 62 and heating elements 64. The crucible is provided with an opening 66 4which is small enough to retain the molten aluminum 68 within the Crucible but on the other hand is large enough to permit the free passage of the filament 2 therethrough. Access to the Crucible is provided by opening 70 in the furnace. A chimney 72 is provided over the Crucible and an argon sweep is fed to the chimney through inlet 74 to reduce the atmospheric contamination in the molten aluminum bath and in the coating achieved on the wire.

Although in various experiments the wire has been drawn both up through the melt and downward, the more uniform coatings have thus far been achieved with the upward draw method.

Various means of applying the overcoat were utilized and a number of different matrix materials lwere utilized, as may readily be seen from the following examples.

EXAMPLE I In a reactor of the type illustrated, utilizing a 61/2 inch long reactor formed from 9 mm. Pyrex tubing, and a reactant gas composition comprising 15.3 mol percent methane, 23.4 mol percent methyldichlorosilane and 61.3 mol percent hydrogen, a silicon carbide coating was produced on boron filaments at a rate of 760 feet/hour.

Fibers thus produced were drawn through a bath of molten aluminum held at a temperature of 10100 C., in an argon atmosphere. The drawing rate, upward through the melt, was 112 ft./min. A continuous overcoat of 0.2 mil thickness was provided on 4.1 mil fiber under these conditions.

EXAMPLE II Other silicon carbide-coated boron filaments were drawn through a melt of 2024 aluminum alloy (nominal coniposition 4.5% copper, 1.5% magnesium, 0.6 manganese, balance aluminum) at a rate of 52 ft./min., the melt temperature being held at 900 C. in an argon atmosphere. The results were similar to those attained with the pure aluminum melt.

EXAMPLE III Satisfactory overcoats of magnesium on silicon carbide coated boron filaments were made by dipping in a melt of magnesium held at 720 C.

EXAMPLE IV Titanium was applied to silicon carbide-coated boron by deposition from titanium iodide on a hot wire held at a maximum of 1100" C., in a retort ata pressure of approximately 5 mm. mercury absolute. The retort in the area of the solid titanium iodide was heated to approximately 230 C.

The results of the various tests performed in terms of fiber properties are detailed in the following table.

TABLE l Ultimate tensile Diameter, strength Fiber i mils p.s.i.

Silicon carbide-coated boron 4. 05 405, 000 Actual fiber with titanium overcoat 4. 60 330, 000 Theoretical fiber with titanium overcoat 4. 60 390, 000

Silicon carbide-coated boron 4. 40 570, O00 Actual fiber wit-h magnesium overcoat 4. 75 414, 000 Theoretical fiber with titanium overcoat 4. 75 510, 000

Silicon carbide-coated boron 4 l0 462, 000 4. '20 494, 000

Actual fiber with aluminum overcoat 4. 35 459, 000 Theoretical fiber with aluminum overcoat 440, O00

In all of the processes whereby the matrix overcoat is provided, the particular technique utilized will be such as lto produce the optimum coating as to quality and thickness. Further considerations will dictate that the process selected be characterized by good reproducibility and uniformity of result and not inconsistent with the rate of production of the basic fiber, although the latter requirement may not always be achievable. The coating parameters will naturally vary from material to material and as a function of the coating method, whether plating, vapor deposition or other conventional techniques. In general, however, the temperature utilized in these processes will necessarily be adjusted such that no unfavorable strength deterioration occurs in the basic boron filament and,

hence, the temperature of the filament will normally be held below the crystallization point of the boron.

The particular thickness of the overcoat applied in a given instance will be determined primarily by the usage to which the filament is to be put. Generally, a minimum thickness suicient to provide an overcoat comprising about 10 percent of the ber in terms of cross-sectional area and preferably at least 0.1 mil will be found advantageous.

In calculating the desired maximum thickness of coating among the factors to be considered is the filamentmatrix volume ratio desired in the liber-reinforced article, the maximum boron fill naturally yielding the higher strengths. Furthermore, while normally the material selected for the overcoat will correspond to the matrix material utilized in the ber-reinforced article, this is not a fundamental requirement. As long as the overcoat material and the matrix material subsequently added are compatible both chemically and in terms of physical properties such as metallurgical interbinding, the results in use will generally be satisfactory.

What is claimed is:

1. A composite filament for use in fabrication of fiberreinforced articles comprising:

a. filamentary substrate consisting essentially of amorphous boron;

a first coating on the substrate consisting essentially of stoichiometric silicon carbide;

and a second coating thei'eover consisting of a ductile,

low density metal selected from the group consisting of aluminum, magnesium, titanium and alloys thereof.

2. A composite filament according to claim 1 in which:

the secoiid coating is formed to a minimum thickness corresponding to about 10 percent of the total cross- Sectional area of the filament.

3. A composite filament according to claim 1 wherein the second coating consists essentially of aluminum or alloys thereof.

6 4. In the processes for forming ber-reinforced articles References Cited wherein a plurality of silicon'carbide-coated boron -la- UNITED STATES PATENTS ments are embedded 1n a suitable matrix materlal, the improvement which comprises: 314371511 4/1969 Hough 117-69 prior to the formation of the article, coating the individual fiiaments with a ductile, 10W density metal se- 5 ALFRED L' LEAVITT .Primary Exlmmel lected from the groups consisting of aluminum, mag- I- R- BATTEN, JR ASSlSaflt EXHmlHer nesium, titanium and alloys thereof to a minimum U s C1 X R thickness corresponding to about 10 percent of the total cross-sectional area ofthe filament. 10 117-69, 106, 107.1, 128; 148--6.3; 244-1 

